1. Field of the Invention
This invention relates to a semiconductor device having an integrated circuit constituted by thin-film transistors (hereinafter referred to as TFTs) and to a method of fabricating the same. The invention relates to, for example, an electro-optical device as represented a liquid crystal display panel and an electronic device mounting the electro-optical device as a part. In this specification, the semiconductor device generally stands for such devices that function by utilizing the semiconductor properties. Therefore, electro-optical devices, semiconductor circuits and electronic devices are all encompassed by the scope of the semiconductor device.
2. Related Art
Technology has been developed for fabricating thin-film transistors (TFTs) by using a thin semiconductor film (of a thickness of from about several nanometers to about several hundred nanometers). TFTs have been put into practical use as switching elements of the liquid crystal display devices and, in recent years, it is becoming possible to form a CMOS circuit and an integrated circuit on a substrate such as of a glass.
Active matrix liquid crystal display devices are becoming a main stream of liquid crystal display devices, by arranging pixel electrodes in the form of a matrix and by using TFTs as switching elements connected to the pixel electrodes, in order to meet the demand for realizing a highly fine picture quality. The active matrix liquid crystal display devices can be roughly divided into two; i.e., those of the transmission type and those of the reflection type. In particular, the liquid crystal display device of the reflection type, which does not use back light, has a merit that it consumes electric power in smaller amounts than the transmission-type liquid crystal display device, and is finding an increasing demand as a direct view-type display device for portable data terminals and video cameras.
The active matrix liquid crystal display device of the reflection type selects a state where an incident beam is reflected by a pixel electrode and is output to the outer side of the device and a state where the incident beam is not output to the outer side of the device by utilizing the optical modulation action of the liquid crystals, produces a bright display and a dark display, and, further, combines these displays to display a picture. In general, pixel electrodes in the liquid. crystal display device of the reflection type are formed of an electrically conducting material having a high optical reflection factor such as of aluminum or silver.
In any way, the size of each pixel becomes inevitably more fine as the picture quality becomes more fine. As a result, the ratio of areas occupied by the TFT, source wiring and gate wiring increases in the pixel portion, and the numerical aperture decreases. In order to increase the numerical aperture of the pixels within a specified pixel size, therefore, it is essential to efficiently lay out the circuit elements necessary for constituting the pixel circuit.